Heretofore, in many occasions, Fourier transform mass sepectrometers are adapted mainly for general organic analysis in order to identify an unknown component. Hence, a transmission unit supplying high-frequency electric field, mounted to the Fourier transform mass spectrometer, for forming an electric field for ionizing a gaseous sample has the function for sweeping a whole region of resonant frequency corresponding to a whole region of mass to be measured at a high speed so as to excite all kinds of ions.
Fourier transform mass spectrometer in the above mentioned field, however, it is said rather less often that an unknown component is required to be identified, but it is required in many occasions that the concentration of the particular known components in a mixed gas sample, to be analyzed is required and its temporal drift. In order to achieve such an object, a so-called calibration curve technique is adopted with the attempt to determine the concentration of a certain sample. In other words, a standard gas composed of known concentrations of the components is prepared for a gas components to be measured, and the relationships between the concentrations and an intensities of a spectral peaks measured are determined in advance. At the time of measurement, the concentration of a particular known component of the sample gas is corrected from the spectral peak intensity of the sample gas with reference to the relationships determined in advance. Hence, the necessary condition of accurate analysis is based on the fact that the peak to be measured is not superimposed whatsoever on any peak other than the component to be measured.
When the conventional Fourier transform mass spectrometers are employed in the field as described hereinabove, the conventional transmission unit excites ions which are not required for excitement, so that voltage of a signal to be induced into a receiving electrode of an analyzing cell amounts to a total sum of outputs caused by resonance from all the ions containing the unnecessary ions. As a consequence, an intensity of an ion cyclotron resonance signal of the ion to be induced is so restricted as not to exceed a dynamic range of analog-digital conversion, so that the ion to be measured cannot be excited until the ion cyclotron resonance signal of the ion to be measured becomes to a sufficiently high level.
Next, the conventional Fourier transform mass sepctrometer presents the problem that there is no correlation between a static magnetic field and the frequency to be irradiated. In other words, if a permanent magnet, an electric magnet or the like is employed, not a super-conductive magnet, application of the static magnetic field for a long term causes the irradiating frequency to deviate from the resonant magnetic field, thereby making the desired ion difficult to be excited.
Therefore, the object of the present invention is to provide a Fourier transform mass spectrometer which can solve the problems as described hereinabove and which is capable of making a ratio of the static magnetic field to the irradiating frequency constant so that an ion to be measured can be excited until a resonant signal of the ion becomes sufficiently high.
Another object of the present invention is to provide a compact Fourier transform mass spectrometer capable of mass analysis of a particular kind of ion to be measured in a stable manner for a long term.